It is common practice to add chemicals to sewage in order to disinfect same prior to discharge thereof into the surrounding watershed. Recently, considerable attention has been directed to the on-site generation of such disinfecting materials, especially sodium hypochlorite, in electrolysis cells. Such on-site production is desirable owing to the problems and expense involved in shipping and storing dilute hypochlorite solutions and the danger in handling the more concentrated chlorine gas in urban areas. Such processes are particularly attractive in coastal locations where the salt values of sea water provide an economical source of the hypochlorite precursor, sodium chloride. Inland, electrolytes may be prepared from solid sodium chloride, whether pure or impure.
The electrolysis of sea, brackish, or impure salt water leads to a pronounced problem, however, in the nature of the formation of heavy deposits on the electrode surfaces. It has been recognized, that calcium and magnesium impurities in the impure saline solutions can cause hardness deposits to build-up on the cathode which leads to pronounced increases in operating cell voltages and ultimately these build-up to such a thickness as to impede or prevent electrolyte flow. It has now been found that manganese dissolved in the impure brine solution causes an anodic deposit to form which likewise increases the operating cell voltage and lowers operating current efficiency. Normally, manganese is not present in sea water at the concentrations at which it causes such electrolysis problems. However, water from harbors in industrial areas often contain much higher concentrations of manganese because of pollution by the steel and other industries or by ground water which is high in manganese content. Typically, ocean water contains from 0.001 to 0.01 parts per million manganese. The manganese existing in ocean water is mainly in the form of Mn.sup.++ or MnSo.sub.4. It has been found that the threshold concentration for adverse effects due to dissolved manganese in the electrolyte in cell performance is in the range of 0.01 to 0.02 parts per million. Thus, it is apparent that normal concentration of manganese and sea water are not particularly troublesome. However, as mentioned before, waters from industrialized harbor areas often contain higher concentrations of manganese. Typically, Jamaica Bay, New York contain 0.05-0.2 ppm manganese whereas Osaka City and Tokyo Bay in Japan contain 1-4 ppm and 0.05-0.1 ppm manganese respectively.
Dissolved iron which is normally in the form of Fe(OH).sub.3 also causes deposits to form on the anode. However, these deposits are generally not deleterious to cell operation although they do discolor and interfere slightly with visual observation.
Chemically speaking, it is known to remove iron and manganese from water supplies by oxidizing these cations with air, chlorine, potassium permanganate, chlorite ion, ozone, and the like.
Various other techniques have been proposed for removing the unwanted deposits from the electrode surfaces and these include mechanical removal of the deposits, acid washing of the cell to dissolve the deposits as well as the use of sand filters or sodium alumina silicate to remove iron and manganese prior to entry of the saline solution into the electrolytic cell. However, none of the proposed solutions has been satisfactory. Mechanical cleaning is expensive and time consuming and acid cleaning can cause corrosion problems. Moreover, all the proposals require stopping the operation of the cell which means it is out of service. The use of sand filter is expensive and difficult particularly where the water is very dirty.